Fiber lasers have many industrial applications, such as micromachining, which require laser pulses having a high peak power on the range of tens of kilowatts or having high pulse energies in the millijoule range. In a fiber laser setup, laser pulses with a high peak power can be achieved by subjecting short nanosecond (herein abbreviated ns) pulses with low pulse repetition rates (herein abbreviated PRR) to high gain amplifiers. However, in fiber laser setups where longer duration laser pulses are required, such as in the range of tens of nanoseconds up to hundreds of nanoseconds, as the pulse width is increased, these high gain amplifiers tend to reach gain saturation due to the limit of energy extraction from the gain medium. The gain medium (i.e. the optical fiber) thus reduces the maximum achievable energy for these laser pulses. In general, gain saturation restricts the sustained peak power over longer time durations.
Besides the issue of gain saturation, fiber lasers can exhibit non-linear effects which can interfere with the generated laser pulses and can cause internal damage to the fiber laser. Non-linear effects, such as stimulated Raman scattering (herein abbreviated SRS) and stimulated Brillouin scattering (SBS) occur in fiber lasers due to interactions between the medium of the fiber laser (i.e., fiber optics) and the generated laser pulses. SRS and SBS manifest themselves as additional laser energy travelling inside the fiber laser which may have a wavelength either higher or lower than the wavelength of the generated laser pulses. As the pulse width of the generated laser pulse increases, the above mentioned non-linear effects become more pronounced in fiber lasers, specifically SBS, which generates shorter wavelength pulses travelling backwards along the gain medium. Internal components in the fiber laser can then be burned and damaged and thus the achievable peak power of the generated laser pulses becomes limited. In fiber lasers, pulses having high peak power and a pulse width of longer than about 5 ns are inevitably accompanied by SBS, which usually manifests itself as an abrupt backwards pulse which is amplified to a high peak power level. Such a pulse can often result in internal damage to the fiber laser. Whereas gain saturation and non-linear effects are not causally related, both phenomena occur under similar conditions. Limited gain produces gain saturation (i.e., gain depletion) whereas high peak power provokes non-linear effects.
Methods and systems are known in the art for compensating for non-linear effects in fiber lasers. For example, SBS occurs in a pronounced way in fiber lasers when there is an increase in the interaction between the medium and the generated light pulses. One way for limiting this interaction and thus raising the threshold for SBS is to use optical fibers with larger core diameters. A drawback of such a fiber laser however is that larger core diameters do not enable single mode beams to be generated. In industrial applications using fiber lasers, where pulse shaping is required, such higher modes (i.e., not single mode) limit the beam quality and thus the performance in terms of resolution spot size.
Fiber lasers, especially those designed as master oscillator power amplifiers (herein abbreviated MOPA), usually include a seeder, also known as a seed laser, which is the initial laser in the design whose output is amplified to generate higher peak power laser pulses. SBS becomes more pronounced when the wavelength of the laser pulses generated by the seeder is narrow enough such that a single frequency (i.e. a linewidth) interacts with the optical fiber's refraction index and forms an acoustic shockwave which ignites an SBS pulse. Another known way of limiting the intensity of SBS is to prevent the laser pulses generated by SBS by controlling the spectral characteristics of the seeder. One known technique is to phase modulate the seeder such that its wavelength is modulated rapidly enough, thereby preventing the long interaction between the gain medium and the SBS pulse. Such techniques however involve higher costs and complexity in fiber laser design.
As mentioned above, high peak powers in fiber lasers leads to non-linear effects such as SBS. High intensity levels of SBS pulses can cause laser damage in high power fiber lasers therefore low threshold levels for SBS limit the achievable peak power in such systems. Furthermore, pulse shaping using slowly rising leading edged pulses further reduces the SBS threshold making pulse shaping even harder to achieve. Reference is now made to FIG. 1A, which is a graph, generally referenced 10, showing a rectangular pulse after amplification exhibiting gain depletion, as is known in the prior art. Graph 10 shows the voltage of a laser pulse 15 over time, generated by a fiber laser. An X-axis 12 of graph 10 represents time in nanoseconds, whereas a Y-axis 14 of graph 10 represents voltage in millivolts, substantially representing the peak power of laser pulse 15. Laser pulse 15 was generated by a seeder as a rectangular pulse and was amplified in a gain amplifier for increasing its peak power. As seen, laser pulse 15 peaks at a peak 16 but then quickly tapers off in a downward slope 18 and reaching a local minimum 20 before ending. Downward slope 18 is due to saturation of the medium, also known as gain depletion.
Reference is now made to FIG. 1B, which is a graph, generally referenced 40, showing an SBS generated pulse produced by amplifying a linear rising pulse, as is known in the prior art. Graph 40 shows the voltage of two laser pulses over time, generated by a fiber laser. An X-axis 42 of graph 40 represents time in nanoseconds, whereas a Y-axis 44 of graph 40 represents voltage in millivolts, substantially representing the peak power of the laser pulses. A first laser pulse 46 represents a linear rising pulse 50 produced by a seeder in a fiber laser. A second laser pulse 48 represents the amplification of first laser 46 in a fiber laser, where second laser pulse 48 shows the SBS driven laser pulse. As seen in a section 52, second laser pulse 48 shows extremely unstable, noisy behavior demarcated by local peaks 54. Local peaks 54, which represent significantly high peak power compared to the peak power of linear rising pulse 50, travel backwards in a fiber laser thus endangering the laser and its components. Linear rising pulse 50 is thus not a good candidate for pulse shaping at high peak power.
What is needed is thus a fiber laser configuration in which pulse shaping at high peak power can be achieved while significantly reducing any limiting effects of non-linear effects in fiber lasers such as SBS.